The present invention relates generally to the field of paper making and, more particularly, to the dryer section of a paper machine.
Specifically, the present invention relates to apparatus for ventilating one or more pockets in a multi-cylinder dryer of a paper machine, the multi-cylinder dryer being of the type wherein guide rolls are situated in spaces between adjoining drying cylinders and wherein a wire or felt has a run guided by the guide rolls so as to include free runs between the drying cylinders and the guide rolls. Further, the pocket ventilating apparatus comprises apparatus by means of which air is blown through the inlet or incoming free wire run into a respective pocket within the multi-cylinder dryer and wherein air is removed from the respective pocket at least partly through the following or outgoing free wire run after the guide roll.
Conventional multi-cylinder dryers in paper machines generally include two rows of drying cylinders situated one above the other and wherein a paper web runs from cylinders in one row to cylinders in another row in open draws. An upper wire or felt and a lower wire or felt is generally provided in connection with the upper and lower cylinder rows respectively, which wires or felts are guided by guide rolls situated in the spaces between adjoining drying cylinders so that the wires or felts press the web against the surfaces of the respective cylinders in the respective rows. Pockets are formed in such a multi-cylinder dryer by the open draws of the web, the free or uncovered surfaces of the drying cylinders and the wires or felts guided by the guide rolls. These pockets are usually open at their transverse ends but otherwise closed. The ventilation of these pockets is an important factor in determining the drying capacity of the multi-cylinder dryer and the uniformity of the drying effects.
In recent years, paper machine speeds have steadily increased which has resulted in the shortening of open draws of the paper web in order to reduce the risk of web breakage, especially due to fluttering. On the other hand, the shortening of the web open draws has resulted in a corresponding increase in the difficulty of ventilating the pockets of the multi-cylinder dryer since a shortening of the web free draw reduces the size of the pockets. Moreover, even higher requirements are imposed in the ventilation of the pockets since the time during which the web remains in association with any one pocket have become shorter as the paper machine speeds have been increased.
Several different arrangements for ventilating pockets in multi-cylinder dryers have been suggested wherein air is blown through the felts or wires into the respective pockets. In these arrangements, the ventilation air is removed through the transverse open ends of the pockets and/or through the free runs of the felts or wires.
With respect to the prior art pertaining to the present invention, reference is made by way of example to U.S. Pat. Nos. 3,388,479, 3,427,727, applicants' assignee's U.S. Pat. No. 3,668,787 and Finnish Pat. No. 45,584, and Finnish Pat. No. 45,363 in the name of Ab Svenska Flaktfabriken.
U.S. Pat. No. 3,388,479 discloses an arrangement wherein blowing apparatus is provided in proximity to the guide roll of the felt or wire. The blowing member directs jets of air against the free runs of the felt or wire which results in air passing through the runs of the felt or wire between adjoining drying cylinders and the guide roll for the felt or wire. Pocket ventilating apparatus which is substantially similar to that described immediately above is also disclosed in U.S. Pat. No. 3,427,727.
These prior art constructions, however, have been found in practice to have a significant drawback resulting from the fact that the nozzles of the blowing members are situated on both sides of the wire or felt guide rolls, i.e., in proximity to both the incoming and outgoing free runs of the wire or felt, so that the air blown into the pocket can escape from the pockets only through the transverse open ends thereof. This results in a considerable air flow in the transverse direction through the pockets. A transverse air flow can cause several problems, the worst being a fluttering of the edge of the web which causes a considerable increase in the risk of web breakage.
An arrangement is suggested in Finnish Pat. No. 44,331 wherein the ventilation air is introduced into the pockets through the pumping effect of the moving guide roll and wire, from one side of the guide roll into the pocket and, correspondingly, out of the pocket on the other side of the guide roll. This arrangement is not efficient and does not provide sufficiently large quantities of ventilating air since it depends only on the natural pumping effect of the various moving components. Moreover, it has been found that when used with modern wire types, the air flowing into the pocket may, depending upon the location of the wire or felt guide roll, be smaller than the outgoing air flow. This in turn results in a transverse air flow wherein air enters into the pocket from the open transverse ends thereof which in turn results in the edge portions of the paper being dried to an extent greater than the average. Still another drawback of this arrangement is the limited capacity to adjust the drying profile since the air introduced into the pocket tends to become mixed outside the wire.
Thus, the conventional arrangements discussed above are not satisfactory in the respects noted above, the problems essentially being the result of the increasing speeds of modern paper machines and the consequent reduction in the size of the pockets and the lengths of the free runs of the web.
An arrangement is disclosed in Finnish Pat. No. 45,584 wherein air is ejected by means of air which is blown through nozzle slots out of the space surrounding a blow box into a space between the blow box and the wire, from which the air flows onward through the wire. The quantity of air passing through the wire, however, remains relatively small since the positive pressure required to obtain a flow of the air through the wire is only produced within a very limited area. The air jet which is discharged from the second nozzle slot collides with a layer of air running along with the wire resulting in the ejection effect of the jet remaining quite low.